1. Field of the Invention
This invention relates to electrical resistors, both fixed and variable, and method for making the same, and particularly relates to resistors commonly known as "thick film" or "cermet" resistors, wherein a glass matrix including conductive materials is deposited on an insulating substrate. The deposited layer or layers are composed so as to include conductive materials of various types, such as noble metals and/or semiconducting oxides of varying consistencies to provide desired resistance values and electrical characteristics and also to provide conductive paths for purposes of termination, where so desired.
2. Description of the Prior Art
Cermet or thick film type resistors were introduced to the market in the early 1960's. In general, the earliest versions were of the type disclosed in the well-known D'Andrea and Dumesnil U.S. Pat. Nos. 2,924,540 and 2,942,992, respectively. D'Andrea taught the use of a composition containing palladium and silver particles in a glass frit, and the Dumesnil patent disclosure was directed to a particular type of glass frit. Both of the disclosures were concerned with depositing resistive or capacitive layers on a prefired insulating substrate, such as barium titanate or other prefired substrate, which could be glass, porcelain, or other refractory.
Earlier, one Nathan Pritikin was issued U.S. Pat. Nos. 2,910,766 and 3,056,937 in which he disclosed a method of producing an electrical component, such as a resistor, wherein the component was constructed of two sheets of preformed and prefired glass. One of the sheets was grooved at opposite ends to receive conducting leads. The other sheets had on one or both of its principal surfaces the desired electrical element. The two sheets of glass were cemented or otherwise secured together in face-to-face relationship, whereby the leads were firmly held in place between the two sheets of glass and in contact with the resistive layer. The preferred embodiment of Pritikin was stated to be one that had the resistance element on one of the concealed surfaces only. It is to be noted that Pritikin disclosed a glass substrate which in effect is a prefired substrate for supporting a resistor film and a terminal cover member. All of his operations were done separately.
Other patents have issued from time to time in the cermet or thick film field but, in the main, these patents have related to variations in metallic constituents and differing glass frits or fluxes to provide higher or lower resistance values, better TCR's (Temperature Coefficient of Resistance), lower current noise and other refinements directed to specific applications and functional specifications. These are exemplified, for instance, in the well-known Place et al U.S. Pat. Nos. 2,950,995 and 2,950,996, as well as the so-called "Birox" thick film glass containing bismuth as taught in U.S. Pat. No. 3,816,348 granted to Popowich.
The Buzard et al U.S. Pat. No. 3,648,363 introduced a cermet resistor, wherein conductor material in the form of a silver, palladium glass frit was first deposited upon a prefired aluminum substrate. A pliable, self-supporting film of resistive material was attached to the conductive layer and the entire unit was fired to mature each of the conductive and resistive layers.
Pritikin U.S Pat. No. 2,796,504 also suggested simultaneously curing conductive and resistive layers of thermosetting plastic material. These layers were supported, however, with a backing of previously cured thermoset layers. A similar technique was disclosed in the U.S. Pat. No. 2,745,931 granted to Heibel. Heibel also used plastic thermosetting material supported by means of a fibrous tape of paper or textile.
A co-firing technique for cermet type resistors is suggested by Cocca in U.S. Pat. No. 3,699,650. However, in this case, only a resistive layer on a prefired substrate and protective glass coating are the only co-fired elements.
A glass substrate formed with suitable binder surrounding embedded leads was disclosed in Loose U.S. Pat. Nos. 3,584,379 and 3,626,353. The substrate and leads were fired together but, here again, the resistive layer was applied separately on the external surface of a prefired body.
It will be apparent that in each of the prior art devices, the substrate is separately fired and is usually of high temperature material, such as steatite or alumina, and of a configuration requiring relatively complex forming and terminating procedures.